Yersinia

Overview

• Cause intestinal disease in animals and are important zoonoses
• 10 species of which Y. pestis, Y. pseudotuberculosis and Y. enterocolitica are pathogenic to animals and humans; Y. pestis is the most pathogenic
• Rodents provide a reservoir of Y. pestis, which is the cause of human plague; fleas transmit the infection to other animals and humans
• Y. pseudotuberculosis and Y. enterocolitica reside in the intestine of domestic and wild animals and birds
• Birds may cause mechanical transfer of the organisms

Characteristics

• Enterobacteria, but grow more slowly and at lower temperatures than other enterobacteria
• Gram negative, non-spore forming, facultative anaerobes - rods or colibacilli
• Non-lactose fermentors
• Facultative intracellular pathogens
• Show bipolar staining in Giemsa-stained smears from animal tissue
• Pathogenic strains identified by serotyping and biotyping

Pathogenesis

• Y. enterocolitica and Y. pseudotuberculosis enter the intestinal mucosa via M cells of the Peyer's patches
• Engulfed by macrphages in the mucosa
• All three invasive species are facultative intracellular organisms and grow inside macrophages
• Plasmid and chromosomal-encoded virulence factors required for survival and multiplication in macrophages
• Survive in phagolysosomes and do not interfere with degranulation or lysosomal fusion
• Resistant to macrophage killing mechanisms
• Antiphagocytic proteins secreted by the organisms interfere with host neutrophils
• Y. pestis is more invasive than the other species and also possesses and antiphagocytic capsule and a plasminogen activator which aids systemic spread; endotoxin also contributes to its pathogenicity
• Transport within macrophages to mesenteric lymph nodes
• Replication in lymph nodes and development of necrotic lesions, with neutrophil invasion
• The bacteria destroy the macrophages causing septicaemia

Clnical infections

• Yersinia pestis
  • Cause of bubonic plague in humans, transmitted via fleas from infected rats
  • Not a significant veterinary disease
  • Disease in rats and other rodents similar to the disease in humans
  • Bubonic form can lead to the pneumonic form, which is highly contagious and usually fatal
  • Humans and domestic and wild animals incidental hosts
  • Plague has rarely been reported in dogs, cats, camels, elephants, deer
  • Cats can acquire the disease from ingesting dead rodents, and show lymphadenopathy and abscesses
  • Fever, lethargy, swelling and abscessation of lymph nodes particularly in head and neck region
  • 50% mortality if not treated
  • Possesses 3 plasmids, 2 of which are unique to this species; these encode an endotoxin, and coagulase and fibrinolytic activity
• Yersinia pseudotuberculosis
  • Less virulent than Y. pestis but closely related
  • Mainly infect in animals
  • One plasmid, required for virulence
  • Sporadic cases of pseudotuberculosis in animals and man
  • Wild birds and rodents provide a reservoir of infection by harbouring the the pathogen in their intestinal tract
  • Sources include food and water contaminated by faeces
  • Multiplication in macrophages leads to granuloma formation
  • Granulomas occur in the gut wall and mesenteric lymph nodes